Electro-optic modulators (EOM) are commonly used in optical communication networks. A phase-modulating EOM may be used in a Mach-Zehnder interferometer to modulate the amplitude of an incoming optical signal. As is known, Mach-Zehnder based opto-electronic modulators have a relatively high power consumption, are large and require a high drive voltage.
Improving the bandwidth-density product in an integrated silicon photonic system requires a corresponding improvement in the performance of the optical modulator disposed in such a system. Optical modulation in conventional optical ring modulators is achieved either by varying the coupling level or by changing the index of refraction of the ring, either by injecting excess minority carriers in the associated PIN junction or by changing the reverse bias voltage applied to the PN junction. The change in the index of refraction causes a change in the optical path length of the ring, in turn changing the resonance frequency of the ring.
Conventional optical ring modulators are susceptible to thermal fluctuations. As the Q of the ring increases, which is desirable for lower power consumption, sensitivity to thermal fluctuations also increases. Thermal fluctuations of an optical ring modulator may be caused by ambient thermal noise as well as data-dependent self-heating of the device. An optical ring modulator (referred to alternatively herein as modulator) absorbs incoming light differently for 1's and 0's, thus causing the temperature of the modulator to depend on the incoming data pattern.
On-chip resistive heaters are conventionally used to compensate ambient thermal fluctuations and data-dependent self-heating. In order to reduce the required power for thermal tuning of the ring modulator, the heat capacity of the device may be decreased. One way to achieve that is by under-etching the photonic IC's substrate.
For the same amount of absorbed heat, the lower the heat capacity of the modulator, the higher is the temperature fluctuations. Therefore, reducing the heat capacity exacerbates data-dependent self-heating. Data-dependent self-heating requires relatively fast thermal tuning which is difficult to achieve with resistive heaters.